The plans for the post-invasion battle had not been worked out in detail. Each Army was expected to advance towards its own objectives; boundaries between the two armies were fixed. In the first two days progress was excellent, capturing Vizzini in the west and Augusta in the east.

However resistance in the British sector then stiffened. Montgomery persuaded Alexander to shift the boundaries so that the British could by-pass the resistance and retain the key role of capturing Messina, while the Americans were given the role of protecting and supporting their flank. Patton sought a greater role for his army, and decided to try to capture the capital, Palermo. After dispatching a 'reconnaissance' toward the town of Agrigento which succeeded in capturing it, he persuaded Alexander to allow him to continue to advance. Alexander changed his mind and countermanded his orders, but Patton claimed the countermand was 'garbled in transmission', and by the time the position had been clarified Patton was at the gates of Palermo.

The fall of Palermo inspired a coup against Mussolini, and he was deposed from power. Although the removal of Italy from the war had been one of the long-term objectives of the Italian campaign, the suddenness of the move caught the Allies by surprise.

After Patton's capture of Palermo, with the British still bogged down south of Messina, Alexander ordered a two-pronged attack on the city. Patton became obsessed with the idea of reaching Messina before the British, writing "This is a horse race in which the prestige of the US Army is at stake.". The Axis, now effectively under the command of German General Hans Hube, had prepared a strong defensive line, the 'Etna Line' around Messina, that would enable him to make a progressive retreat while evacuating large parts of his army to the mainland. Patton began his assault on the line at Troina, but it was a lynchpin of the defense and stubbornly held. Despite three 'end run' amphibious landings the Germans managed to keep the bulk of their forces beyond reach of capture, and maintain their evacuation plans. Elements of the US Third Infantry Division entered Messina just hours after the last axis troops boarded ship for Italy. However Patton had won his race to enter Messina first.